In a continuous casting operation, molten metal is throttled through a valving system from a tundish and into and open-ended mold. The steel, which has solidified on its surface but remains liquid inside, is withdrawn from the mold at ideally the same rate at which it is being throttled though the valving mechanism. Several well known valve systems have been devised to throttle the flow of molten metal in a continuous casting process. One such system in common use today is described in U.S. Pat. Nos. 4,063,668 and 4,199,087.
Since the refractory parts of such valve systems are being constantly exposed to molten metal, these parts deteriorate and need to be replaced. The periodic replacement of these wear parts necessitated by their deteriorations thus presents itself as a limiting factor in the length of continuous operation and operating efficiency of these systems. The system described in the above patents presents an advantage over other systems in that both the lower plate as well as the throttling plate are replaceable in the course of a continuous casting operation, thus extending the time in which the operation may continue uninterrupted and improving the efficiency of operation.
While the above mentioned system extends the period of continuous operation by eliminating the throttling plate, the lower plate, and the submerged nozzle as the limiting wear factors, a secondary limiting factor has arisen which the present invention is intended to also alleviate. That limitation concerns the joint between the lower plate and the submerged nozzle which introduces the metal into the open-ended mold.
In the above mentioned system, bolts attach the lower plate of the valving mechanism to the submerged nozzle. As these components are assembled prior to being inserted into an operating slide gate assembly, the relative tightness of the connection will invariably change since the lower plate, the nozzle and the attaching bolts all have differing thermal expansion characteristics. No matter how tightly or loosely the lower plate is sealed against the nozzle in pre-assembly, the temperatures of operation will cause uneven expansion between the metal bolts and the refractory plate with undesirable attendant results. Excessive expansion of the bolts can cause a loss of the sealing relationship between the lower valve plate and the nozzle. This loss of a sealing relationship allows atmospheric gases to enter into the molten stream, thus causing undesirable impurities in the finished product. On the other hand, if the bolts are initially tightened too tightly to account for the relative expansion, the refractory components are subject to cracking, which also has the effect of allowing atmospheric gases to enter the molten stream and, more critically, may result in a complete break-up of the refractory parts and an uncontrolled spillage of molten metal. Access to the bolts for adjustment during operation is difficult and dangerous since the operation must come in close proximity to the stream of molten steel. Moreover, precise uniform adjustment required of the several attachment bolts to avoid uneven stress concentrations is virtually impossible to obtain under operating conditions. It is to be further noted that these changes occurring at this joint are also effecting the rate of flow of the molten metal through the valving system, thus making the ultimate task of continuous casting more difficult.
U.S. Pat. No. 4,199,087 partially recognizes this problem and attempts to alleviate it by injecting an inert gas into the molten stream at the joint between the lower plate and the submerged nozzle. This presented solution is inadequate in that it fails to address the variation of conditions acting upon the joint in question which may cause cracking and/or leaking in spite of this suggested stop-gap measure.